Well sealing method and apparatus

ABSTRACT

An apparatus for forming a plug in a casing includes a body of plug material and a carrier for insertion into a casing. The carrier supports the body of plug material. The carrier includes a mandrel and at least two circular flanges spaced apart along the mandrel. The carrier also includes a heater for heating the mandrel. The mandrel is heated to a temperature above the melting point of the material and the plug material slumps into the casing between the at least two circular flanges. The at least two circular flanges force the expanded solidifying plug material against the casing which aids the transfer of heat between the mandrel and the plug material, and resists creep of the solidified material along the casing.

This application is the National Phase of International ApplicationPCT/GB01/04260 filed 24 Sep. 2001 which designated the U.S. and thatInternational Application was published under PCT Article 21(2) inEnglish.

The present invention relates to a method and apparatus for sealingunderground components to prevent leakage of for example hydrocarbonfluids from those components.

In the oil and gas extraction industries, abandoned wells have to beplugged to keep the contents of deep high pressure environments whichcommunicate with those wells from invading levels at or adjacent thesurface. Plugs can be inserted at any point in a well, for exampleadjacent the surface or at a substantial depth. Typically, plugs areformed by injecting cement or resin into the well so as to fill forexample a fifty metre length of the well. Experience has proved howeverthat such plugs are not particularly reliable and often leak.

The known plugs tend to leak for a variety of reasons. Firstly, as thewell wall is typically not particularly clean and is also covered with ahydrocarbon film, it is difficult to produce a reliable contiguous seal.Often a contiguous seal of only a metre or so in length is formed with aplug fifty times that length. Furthermore, as cement and resin basedplugs solidify they contract which tends to open up a gap between theplug and the well wall. Although when a plug is initially inserted theremay be little dynamic pressure in the well, after the plug is in situsubstantial pressures can build up and as a result a plug which appearsinitially to be working satisfactory may subsequently be found to leak.If hydrocarbons leak past the plug contamination of the surfaceenvironment or for example a sub-surface aquifer can result. It is wellknown in the industry that a significant proportion of abandoned wellsleak. As a result leaking abandoned wells often have to be re-pluggedwhich is an expensive and time consuming operation.

It is known to form temporary seals in chemical plants by freezing wateror other fluids in the plant. Such plugs are used for example to sealpipes whilst work is conducted upon systems connected to those pipes.The advantage of this approach is that the system does not need to bedrained prior to work being initiated. Ice plugs do form reliable sealsbut require continuous cooling given that the sealed pipe will normallybe at a temperature above melting point of ice.

It is an object of the present invention to provide a method andapparatus for plugging a well which obviates or mitigates the problemsoutlined above.

According to the present invention, there is provided a method forforming a plug in a well, wherein a length of the well is filled with amolten material the melting point of which is higher than thetemperature within the well and which expands as it solidifies.

The material may be a metal alloy, for example a low-melting pointbismuth-containing alloy such as “Rose's metal”, “Kraft's alloy” or“Homberg's alloy”. The bismuth-containing alloy may be doped withsodium. Such alloys expand upon solidification and thus once depositedin a well they lose heat into the surrounding environment, solidify, andin solidifying expand to form a secure plug within the well.

The material may be delivered to the well in a molten state. For examplea canister of molten material may be lowered to the intended site of theplug and opened either by remote control or deliberate rupture of thecanister. For sodium doped material the doping may be achieved by addingan ingot of sodium to the material when in a molten state when the alloyis first prepared. The sodium is added when the alloy is firstmanufactured, not “down-hole” when it is used to form the plug.

Alternatively, the material may be delivered to the well in a solidifiedstate, subsequently melted in the well, and then allowed to solidify.For example, the material could be delivered in granular form, forexample in a carrier fluid. The granular material could then be meltedin any suitable manner, thereafter cooling to form a solidified plug.The granular material could be melted by delivering it in a first fluid,then adding a second fluid which when mixed with the first elevates thetemperature to above the melting point of the granular material. Thegranular material then melts, and subsequently cools to form asolidified plug. The first fluid could be for example inhibitedhydrochloric acid whereas the second could be for example caustic soda.

As a further alternative the plug material may be delivered to the welland located therewithin mounted in solid form on a carrier. Such acarrier may comprise a chemical heater, for example a “Thermit” mixture,which when ignited provides thermal energy to melt the plug materialwhen it is located at the required well depth. The carrier mayincorporate an engagement means to engage the well casing when inposition. Such engagement means may be arranged to allow insertion ofthe carrier into the well and movement in a down-hole direction thereinbut prevent up-hole movement. This may be achieved by coupling theengagement means to the carrier via a hinge.

Non-chemical methods of melting the plug material could of course beused, for example steam, heated water, electrical resistance heating,frictional heating, sonothermic (sound generated) heating, cavitational(pressure generated) heating, or even simply introducing a solid highthermal capacity mass from which heat is transferred into the previouslydeposited granular material.

As a further alternative, a first component of the material which has amelting point lower than the temperature within the well may bedelivered in a molten state, and a second component may then be added tothe first, the second component mixing with the first and the resultantmixture having a melting point which is higher than the temperaturewithin the well. Thus the first component could be accurately positionedin situ, visually inspected and then converted into a solid plug simplyby pouring the second component into the well so that it mixes with thefirst component.

Once the molten material has solidified and formed a plug it may becapped with a coating material. A preferred example of a suitablecoating material is concrete.

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompany drawings in which:

FIG. 1 is a schematic representation of a conventional well pluggingmethod;

FIG. 2 is a schematic representation of a first method for plugging awell in accordance with the present invention;

FIG. 3 is a schematic representation of a second method for plugging awell in accordance with the present invention;

FIG. 4 is a schematic representation of a third method for plugging awell in accordance with the present invention;

FIGS. 5, 6 and 7 are side views of components used in the third method;

FIGS. 8, 9 and 10 are schematic cross-sections (viewed from the side) ofcomponents used in the third method;

FIG. 11 is a schematic representation of a fourth method for plugging awell in accordance with the present invention; and

FIGS. 12 and 13 are side views of components used in the fourth method.

Referring to FIG. 1, an oil well has a casing 1 within which productiontubing 2 extends axially. In order to plug the well, it is necessary toblock both the annular passageway between the casing 1 and theproduction tubing 2 itself. Conventionally this is achieved by blockingthe casing 1 beneath the production tubing 2 by inserting a packer 3.The space above the packer 3 is then filled with cement 4 to a depthtypically of 50 meters. The cement then solidifies, forming the requiredplug.

The surface of the casing 1 and the surface of the production tubing 2contacted by the cement are not ideal for forming sealing interfaces.Those surfaces are contaminated with hydrocarbon components and otherdeposits. Furthermore, as the cement hardens, it tends to contract sothat in the absence of adhesion between the cement 4 and the casing 1 anannular gap can develop between the cement 4 and the casing 1. Althoughat the time the well is plugged generally there will be little upwardsflow through the well because the seal is initially adequate, over timethe seal can fail so that fluids will migrate up the gap created by sealfailure. As a result unwanted fluid can leak past the plug 4 (shown byarrow 5), risking contamination of the local environment.

Referring now to FIG. 2, this illustrates a first embodiment of thepresent invention. The same reference numerals are used for equivalentcomponents as in FIG. 1. In contrast to the cement plug 4 of FIG. 1however in FIG. 2 a bismuth alloy plug 6 is formed within the casing 1above the packer 3. The length of casing 1 filled with the plug 6 isrelatively small as compared with the length of casing filled by thecement 4 as shown in FIG. 1. Typically the length of casing 1 filledwith the plug 6 is an order magnitude less than that often encounteredwith cement plugs (typically 50 meters). Nevertheless, the arrangementof FIG. 2 provides a reliable plug 6 because the bismuth alloy isinitially molten within the casing 1 but subsequently solidifies andduring solidification expands to tightly engage the casing 1 and theproduction tubing 2. Thus a solid plug 6 expanded metal seals the casing1 and locks together and components contracted by the plug 6.

The bismuth alloy plug 6 could be formed from one of the low-meltingpoint bismuth containing alloys known from the printing industry, forexample “Rose's metal” (melting point 93° C.), “Kraft's alloy” (meltingpoint 104° C.), “Homberg's alloy” (melting point 122° C.) or any alloywhich may be developed to have characteristics suitable for thedown-hole conditions. For example antimony could be used to form highertemperature melting point alloys. Such alloys increase in volume uponsolidification and thus are sometimes referred to as “expanding metalalloys”. The unusual property of expansion upon solidification has beenused to advantage in the printing industry to lock printing blocks intoprinting frames. In the environment to which the present inventionrelates, once installed the bismuth alloy plug 6 will be a permanentfixture given that the stable temperature of the local environmentensure that it remains in a solid state. As the plugs expand onsolidification, they form reliable seals along their entire length.

The solid plug 6 of FIG. 2 can be formed by delivering molten bismuthalloy in a canister or the like which is lowered to just above thepacker 3 and then opened or ruptured as convenient. The released moltenalloy flows to form a solid plug 6 above the packer 3 and then cools tothe local temperature (typically 40-100° C.). Thus the plug 6 solidifiesand expands. Alternatively, the alloy could be delivered via a coiledtubing placement system.

Optionally the bismuth alloy may be doped with sodium. This may beachieved by adding an ingot of sodium to the bismuth alloy when it ismolten during the manufacturing process. It will be appreciated that assodium melts at a low temperature (98° C.) it will melt in the moltenalloy and become dispersed. Other methods of doping such as ionbombardment and pre-doping of the bismuth alloy before it is located insitu are not hereby precluded. The doping of the bismuth alloy may besuch that around 1% of the alloy is comprised of sodium. It ispostulated that doping with sodium serves as an aid in preventing creepof the bismuth alloy plug 6.

As an alternative to delivering molten alloy to the space above thepacker 3, it would be possible to deliver a solid alloy in for examplegranular form and then to melt the alloy in situ using any convenientheating system. FIG. 3 illustrates one arrangement in which thenecessary heating is delivered by an exothermic reaction.

Referring to FIG. 3, this illustrates a second embodiment of the presentinvention. Once again the same reference numerals are used as in FIG. 1where appropriate. The space above the packer 3 however is filled with alayer of bismuth alloy granules 7 immersed in a carrier fluid of forexample inhibited hydrochloric acid 8. The acid 8 cleans the surfaces ofthe down hole components above the packer 3 but does not attack themetal of the down hole components nor the bismuth alloy granules 7. Allof the introduced materials will warm up to the local ambienttemperature. This is too low to cause the granules to melt. A furthercomponent can then be added, that is caustic soda (illustrated by arrow9), which reacts with the hydrochloric acid 8 in an exothermic reactionthat elevates the local temperature to above the melting point of thealloy granules 7. The granules 7 thus melt and coalesce. In mostapplications, the local pressure will be sufficiently high to preventthe reactants from boiling and dissipating the generated heat. The endreaction products of the cooled acid 8/alkali 9 reaction are sodiumchloride and water which are benign to the natural environment. It willof course be appreciated that other exothermic reactions could be used.It will also be appreciated that any other convenient method for meltingthe alloy granules 7 could be used.

Referring now to FIGS. 4-10 a third embodiment of the invention isillustrated using the same reference numbers as above where appropriate.Once again a bismuth alloy plug 6 is formed within the casing 1 abovethe packer 3. In contrast to the first and second embodiments of theinvention, the solid bismuth alloy plug 6 (shown in FIG. 4) is formedfrom an amount of bismuth alloy delivered in solid form on a carrierspool to the required depth within the casing 1.

The carrier spool may comprise 1% manganese steel. The carrier spoolcomprises a tubular mandrel 10. The mandrel 10 has an upper open end.The lower end of the mandrel 10 terminates in a head 11, upon which acylindrical packer 3 (preferably comprising vulcanised rubber including40% acrylonitrile) is mounted. The packer 3 may be mounted on the headby a method which includes a bonding step, thus forming a metallelasticbond. The head 11 defines a frustocone the base of which has a lowerdiameter than that of the packer 3 and which tapers from the uppersurface of the packer 3 to the mandrel 10. The mandrel 10 has aplurality of circular flanges 12 in the form of fins distributed atintervals along its length. The diameter of each fin 12 is approximatelyequal to the diameter of the base of the frustocone 11.

In delivery form (shown in FIG. 6) the solid metal locates along thelength of the mandrel 10 between the head 11 and an upper fin 12,defining a cylinder extending as far as the peripheral edge of the upperfin 12. The metal may comprise, for example, pure bismuth, an admixtureof 95% bismuth and 5% tin or an admixture of 52% bismuth and 48% tin, ineach case the metal may be doped with sodium. In this form the carrierspool is inserted into the casing 1 (packer end first) and lowered tothe required depth.

Thus positioned the bismuth alloy is melted in situ by a heater whichnormally locates within the mandrel 10 (but which is illustrated forclarity in FIG. 7 outside the mandrel 10). The heater defines acylinder, an upper portion of which comprises an ignition source 13 anda lower portion of which comprises a heater element 14. The heaterelement 14 may comprise an admixture of aluminium and iron oxide(thermit mixture). The ignition source 13 may comprise a barium peroxidefuse and an electrical heater. It will be appreciated that other formsof both ignition source 13 and heater element 14 could be used.

Commonly the ignition source 13 is activated using a fuse 15. The fuse15 is preferably disposed in a bore 16 in a threaded cap 17 whichengages a threaded portion 18 of the mandrel 10. The cap 17 may define asimple hollow plug (as shown in FIG. 8) or may include features such asincisions 19 (as shown in FIG. 9) which allow the cap 17 to be engagedby other equipment (not shown) such as a deployment tool. The cap 17 maydefine a stab connector.

Activation of the detonator 13 triggers the heater element 14. Heatproduced from the heater element 14 causes the bismuth alloy supportedon the mandrel 10 to become molten. Combustion/waste gases which may beproduced from the heater element 14 are allowed to be vented by the openend of the mandrel 10 and the cap 17.

The molten bismuth alloy thus slumps into the volume defined by theupper surface of the rubber packer 3 and the casing wall 1 (as shown inFIG. 6).

The frustocone 11 is able to serv as a wedge that drives into theexpanded bismuth alloy plug 6. Thus pressure from the reservoir servesto force the plug 6 against the casing wall 1.

The fins 12 serve three purposes. Firstly the fins 12 aid in forcing theexpanding metal against the casing 1 by minimising axial and promotinglateral expansion. Secondly the fins 12 aid the transfer of heat fromthe heater element 14 to the bismuth alloy. Thirdly the fins 12 aid inreducing creep of the bismuth alloy plug 6 up hole.

Referring now to FIGS. 11 to 13 a fourth embodiment of the invention isillustrated. Once more a bismuth alloy plug 6 is formed within thecasing 1 above a packer 3. The bismuth alloy is delivered using acarrier spool as in the third embodiment. The fourth embodiment differsfrom the third embodiment in that casing engagement means are coupled toa fin 12 on the carrier spool.

The casing engagement means comprises arms 20 a and 20 b coupled by ahinge (not shown) to an upper surface of a fin 12. Although two arms 20a, 20 b are illustrated coupled to one fin 12 it will be appreciatedthat additional arms may be included, either coupled to the same orother fins 12. The non-coupled ends of arms 20 a and 20 b have one or aplurality of casing engagement members (not shown), which may comprisefor example a spike, a tooth, a chisel or another engagement member.

In delivery form the arms 20 a, 20 b are retained within the bismuthalloy so that the carrier spool carrying the bismuth alloy may belocated at the required depth in the casing 1. Activation of the heaterelement 14 causes the bismuth alloy to be melted and thus the arms 20 a,20 b are free to fall into engagement with a portion of the casing 1.

As the bismuth alloy solidifies and expands the arms 20 a, 20 b areurged into a pressured engagement with the casing 1. Thus deployed thearms 20 a, 20 b act as an aid in preventing reservoir pressure inducedcreep of the plug 6 along the well. Indeed any increase in reservoirpressure will merely cause stronger engagement of the engagement meanswith the casing 1.

In a further embodiment of the invention which is not specificallyillustrated, an alloy could be introduced the melting point of which islower than the ambient temperature immediately above the packer 3. Thecomposition of the alloy could then be changed to raise its meltingpoint above the local ambient temperature. For example, alloys are knownwhich have melting points below 40° C. but into which lead can beintroduced to form a mixture the melting point of which is well above40° C. Thus one could envisage the formation of a plug by pouring afirst component into the well so as to form a body of liquid alloyimmediately above a packer, inspecting the deposited liquid alloy toensure that the alloy is safely retained in place, and then simplypouring lead granules into the well such that the granules becomeimmersed in the liquid alloy, causing the combined liquid/alloy leadgranule mixture to form a solid plug.

In a yet further embodiment of the invention which is not specificallyillustrated a further plug may be formed above the plug of solidifiedmaterial. Such a further plug may be used to provide additionalresistance against creep of the plug of scheduled material caused bypressure from the reservoir. As the further plug is not being used toprovide a seal for the well its length need not be of the same order ofmagnitude as conventional plugs described in the prior art. For examplethe further plug may be of the order of 5-10 meters long. A preferredmaterial for the further plug is concrete.

1. A method for plugging a casing using a body of plug material into thecasing on a carrier, the body of plug material having a melting pointhigher than the temperature of the casing to be plugged and expanding onsolidification, the plug material being supported on a mandrel on whicha plurality of circular flanges are spaced apart, comprising: insertingthe mandrel carrying the body of plug material into the casing; heatingthe mandrel to a temperature higher than the melting point of the plugmaterial so that the body of plug material melts to become molten plugmaterial and slumps into the casing between the circular flanges; andcooling the molten material to solidify between the circular flanges,the circular flanges forcing the expanding solidifying plug materialagainst the casing, which aids the transfer of heat between the mandreland the plug material and reduces creep of the plug material along thecasing.
 2. A method according to claim 1, wherein a packer is supportedon a mandrel beneath the circular flanges to prevent molten plugmaterial flowing away from the circular flanges.
 3. The method accordingto claim 1, wherein the plug material is metal alloy.
 4. The methodaccording to claim 3, wherein the plug material is a bismuth-containingalloy.
 5. The method according to claim 3, wherein the plug material isdoped with sodium.
 6. The method according to claim 1, further includingadding a further plug above the plug material when solidified.
 7. Themethod according to claim 6, wherein the further plug includes concrete.8. An apparatus for forming a plug in a casing, comprising: a body ofplug material; and a carrier for insertion into a casing to support thebody of plug material, the carrier including a mandrel, at least twocircular flanges spaced apart along the mandrel, and a heater forheating the mandrel, and wherein the plug material has a melting pointhigher than the temperature within the casing and lower than atemperature to which the heater heats the mandrel, the plug materialexpanding when it solidifies, and the plug material being carried on themandrel such that if the mandrel is heated within the casing to atemperature above the melting point of the plug material, the plugmaterial slumps into the casing between the at least two circularflanges and the at least two circular flanges forcing the expandedsolidifying plug material against the casing, which aids the transfer ofheat between the mandrel and the plug material, and resists creep ofsolidified material along the casing.
 9. The apparatus according toclaim 8, including a packer supported on a bottom end of the mandrel.10. The apparatus according to claim 8, further including arms retainedwithin the plug material, the arms being arranged to fall intoengagement with a casing which the plug material is melted.
 11. Theapparatus according to claim 8, wherein the plug material is a metalalloy.
 12. The apparatus according to claim 11, wherein the plugmaterial is a bismuth-containing alloy.
 13. The apparatus according toclaim 11, wherein the plug material is doped with sodium.
 14. Theapparatus according to claim 8, including a frustoconical head at alower end of the mandrel beneath the circular flanges, the frustoconicalhead tapering upwards.
 15. The apparatus according to claim 8, whereinthe mandrel is tubular, and the heater is received within the tubularmaterial.